1. Field of the Invention
The present invention relates to an image forming apparatus and a turn-on time correction method.
2. Description of the Related Art
An image forming apparatus provided with a Vertical Cavity Surface Emitting Laser (VCSEL) of which turn-on is controlled according to image data may have the following cases. That is, when the VCSEL is turned on, it may take time for the light emitted therefrom to reach a specified amount of light because of its slow rise time, or the specified amount of light may be obtained after overshoot of the amount of light. In the former, image density decreases, while in the latter, the image density increases. Therefore, if the response characteristic changes for each of a plurality of light sources in the VCSEL, this causes uneven density or irregular color.
From the reasons above, there is disclosed a technology for obtaining an image without uneven image density by emitting light sources to detect an integrated light amount and controlling the light emitting pulse width or controlling the voltage and current to drive the light sources based on the result of detection (see, for example, Japanese Patent Application Laid-open No. 2003-039722, Japanese Patent Application Laid-open No. H9-200522, and Japanese Patent Application Laid-open No. H11-078117).
More specifically, an image forming apparatus according to Japanese Patent Application Laid-open No. 2003-039722 is configured so that a detector detects the integrated light amount of laser diodes (LDs) for a plurality of levels of pulse widths for each LD and a controller controls the bias and the pulse width for each LD based on the result of detection by the detector.
An image forming apparatus according to Japanese Patent Application Laid-open No. H9-200522 is configured so that a monitor circuit monitors an emission intensity of LD and a correction circuit integrates outputs of the monitor circuit to determine an integrated light amount and controls an integrated light amount based on the determined integrated light amount.
An image forming apparatus according to Japanese Patent Application Laid-open No. H11-078117 is configured so that a part of light beam is guided to a photoelectric conversion device through a half mirror and the photoelectric conversion device converts a value of light accumulated at each light-emitting points, entering during a period specified by a timing signal received from a control device, to an electrical signal. A controller refers to the accumulated light value signal to adjust the voltage and current to drive the light source or to adjust an optical pulse width so as to be equal to the reference level. This results in correction of variations among the light-emitting points and of changes due to temperature, thus improving image quality.
However, the response characteristic of each of the light sources changes caused by not only a factor such as temperature but also the amount of light to be emitted, and further changes also caused by a turn-off time before starting of the emission. When an image is actually formed, the amount of light is changed by process control, and the turn-off time is also changed caused by the image data. As a result, even if the process control is implemented to change the amount of light in order to improve the image quality, the image quality decreases depending on the change in the response characteristic of the light source according to the change in the amount of light. In addition, the change in the response characteristic of the light source according to the turn-off time that changes according to the image data may cause uneven image density.